Oxygen-rich blood from the lungs comes to the thin-walled upper chamber of the heart on the left, the left atrium. It relaxes while collecting blood. Then, it contracts to transfer the blood to the left ventricle.
When the left atrium contracts, the next chamber, the left ventricle relaxes so that the blood is transferred to it. When the muscular left ventricle contracts in its turn, the blood is pumped out to the body.